Comparative Proton Coupled Electron Transfer at Glassy Carbon and Boron‐Doped Diamond Electrodes
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-01-08 |
| Journal | ChemElectroChem |
| Authors | Shane P. O. Neill, Adrià Martínez‐Aviñó, Charlie Keene, S Mohammed Hassan, Catriona M. Houston |
| Institutions | University College Dublin, University of Lincoln |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study compares the Proton Coupled Electron Transfer (PCET) properties of anthraquinone (AQ) immobilized on sp2 Glassy Carbon (GC) versus sp3 Boron-Doped Diamond (BDD) electrodes using an identical solid-phase synthesis route.
- Comparable Surface Density: The diazonium reduction and subsequent coupling achieved highly similar surface densities of anthraquinone on both substrates (~1.0 x 1014 molecules cm-2), confirming equal synthetic efficiency.
- Substrate-Dependent pKa Shift: The apparent pKa1 of the immobilized AQ was found to be highly dependent on the substrate material: 9.1 ± 0.2 on GC (more basic than solution) but 6.6 ± 0.2 on BDD (more acidic than solution).
- Slower Kinetics on BDD: Electron transfer kinetics (KET) were significantly slower on BDD (0.41 s-1) compared to GC (4.5 s-1), consistent with BDD’s lower charge carrier density.
- Dielectric Constant Discrepancy: The dramatic pKa difference is attributed to the effective dielectric constant (εFilm) of the immobilized layer. Assuming a 1.5 nm film thickness, εFilm was calculated as 68 ± 3 for GC, but only 6.7 ± 0.4 for BDD (a 10x reduction).
- Engineering Caution: The results demonstrate that established sp2 surface modification methodologies cannot be directly transferred to sp3 BDD without careful re-evaluation of the resulting layer’s fundamental chemical and electrochemical properties.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Surface Density (GC) | 1.0 ± 0.1 x 1014 | molecules cm-2 | Anthraquinone moiety |
| Surface Density (BDD) | 0.9 ± 0.3 x 1014 | molecules cm-2 | Anthraquinone moiety |
| Apparent pKa1 (GC) | 9.1 ± 0.2 | N/A | pH dependence of midpoint potential |
| Apparent pKa1 (BDD) | 6.6 ± 0.2 | N/A | pH dependence of midpoint potential |
| Electron Transfer Rate (KET) (GC) | 4.5 | s-1 | Laviron analysis (pH 7 PBS) |
| Electron Transfer Rate (KET) (BDD) | 0.41 | s-1 | Laviron analysis (pH 7 PBS) |
| Film Capacitance (CFilm) (GC) | 3.7 x 10-5 | F cm-2 | Electrochemical Impedance Spectroscopy (EIS) |
| Film Capacitance (CFilm) (BDD) | 4.5 x 10-6 | F cm-2 | Electrochemical Impedance Spectroscopy (EIS) |
| Effective Dielectric Constant (GC) | 68 ± 3 | N/A | Calculated assuming 1.5 nm film thickness |
| Effective Dielectric Constant (BDD) | 6.7 ± 0.4 | N/A | Calculated assuming 1.5 nm film thickness |
| PCET Potential Shift (2e/2H+) | 59 | mV/pH unit | High pH regime |
| PCET Potential Shift (2e/1H+) | 30 | mV/pH unit | Low pH regime |
| GC Electroactive Area | 0.19 ± 0.05 | cm2 | Determined via ferrocenemethanol CV |
| BDD Electroactive Area | 0.068 ± 0.003 | cm2 | Determined via ferrocenemethanol CV |
Key Methodologies
Section titled “Key Methodologies”The anthraquinone layer was immobilized on both GC and BDD electrodes using a three-step solid-phase synthesis route:
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Electrode Pre-Treatment:
- Mechanical Cleaning: GC electrodes polished using P1200 silicon carbide paper, followed by 1.0 µm alumina powder.
- Electrochemical Cleaning: Electrodes subjected to 20 cycles in H2SO4 solution (0 V to 1.5 V vs. SCE) at 0.1 V s-1.
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Linker Grafting (Diazonium Reduction):
- Reagent: 4-(((tert-butoxycarbonyl)amino)methyl)benzene diazonium tetrafluoroborate (Boc-protected linker).
- Conditions: Solution of 0.01 M linker and 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile.
- Procedure: Potential swept electrochemically between 0.6 V and -1.0 V vs. SCE for 5 cycles at 0.05 V s-1, forming a covalent aryl bond to the carbon surface.
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Boc Deprotection:
- Reagent: 4.0 M HCl in dioxane.
- Procedure: Electrode submerged for 4 hours to remove the Boc protecting group, yielding a free amine surface.
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Anthraquinone Coupling (Amide Bond Formation):
- Reagents: 0.1 M Anthraquinone-2-carboxylic acid, 0.1 M N-(-3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC), and 0.06 M N-hydroxysuccinimide (NHS).
- Solvent: Dimethylformamide (DMF).
- Procedure: Electrode suspended in the coupling solution for 16 hours, covalently linking the anthraquinone moiety to the surface amine via an amide bond.
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Electrochemical Analysis:
- Kinetics (KET): Determined using the Laviron procedure by measuring peak potential shifts as a function of scan rate (0.1 V s-1 to high V s-1).
- Capacitance (CFilm): Determined using Electrochemical Impedance Spectroscopy (EIS) in 1 M KCl PBS solution (1 kHz to 3 MHz frequency range, ± 10 mV amplitude, bias -0.8 V).
Commercial Applications
Section titled “Commercial Applications”The findings regarding the controlled modification of BDD surfaces and the resulting electrochemical properties are relevant to several high-performance electrochemical and sensor applications:
- High-Stability pH Sensors: Anthraquinone moieties are established pH probes. The ability to precisely control the apparent pKa (shifting it from 9.1 on GC to 6.6 on BDD) allows for the engineering of sensors optimized for specific, narrow pH ranges, particularly in acidic or neutral environments.
- Bio-Electrochemical Interfaces: BDD electrodes offer a wide potential window and superior resistance to biological fouling compared to GC. This makes BDD-based AQ films ideal for use in complex biological media or implantable sensors where stability and cleanliness are paramount.
- Electrocatalysis: Quinones are used as catalysts, notably in the synthesis of hydrogen peroxide (H2O2). The observed differences in electron transfer kinetics (KET) and local environment (dielectric constant) provide critical design parameters for optimizing BDD-based electrocatalysts for specific reaction pathways.
- High-Precision Redox Probes: BDD’s inherently low background current and wide working potential window enhance the signal-to-noise ratio for immobilized redox species, enabling high-precision measurements in fundamental electrochemistry and analytical sensing.
- Surface Engineering Validation: The study provides a crucial warning for engineers developing surface functionalization protocols, emphasizing that methods optimized for sp2 carbon must be rigorously re-validated when transitioning to sp3 diamond materials.
View Original Abstract
Abstract The surface modification of carbon electrodes is an area of great interest in both fundamental and applied electrochemistry. Herein we demonstrate a reliable route for the modification of sp 3 boron‐doped diamond electrodes through a diazonium reduction and subsequent solid phase synthesis to produce a stable, immobilised layer of surface‐bound anthraquinone. The electron transfer kinetics, surface coverage, and p K a of the immobilised anthraquinone were investigated and compared to those of anthraquinone immobilised via an identical synthetic route onto a glassy carbon sp 2 interface. The p K a of anthraquinone was found to be 9.1 on glassy carbon but 6.6 on boron‐doped diamond. Differences in p K a were observed despite the use of identical surface modification strategies and the achievement of comparable surface densities for both types of electrode, and are attributed to the differing dielectric properties of the surface‐modified layers atop either an sp 2 or sp 3 interface. These results highlight how the underlying substrate can greatly influence the fundamental chemical and electrochemical properties of immobilised molecules, as well as the need for caution when applying well‐established sp 2 solid phase synthesis methodologies to sp 3 substrates.